Body water
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In medicine, body water is all of the water content of the human body. A significant fraction of the human body is water. Lean muscle tissue contains about 75% water by weight. Blood contains 95% water, body fat contains 14% water and bone has 22% water. Skin also contains much water. The human body is about 60% water in adult males and 55% in adult females.
In diseased states where body water is affected, the compartment or compartments that have changed can give clues to the nature of the problem. Body water is regulated by hormones, including anti-diuretic hormone (ADH), aldosterone and atrial natriuretic peptide.
There are many methods to determine body water. One way to get a simple estimate is by calculation.
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[edit] Calculation of body water
In individuals of normal weight, water is abundant in most parts of the body, except in adipose tissue (fat). These calculations are for adults of average build, and are inappropriate for obese or overly muscular people. These proportions are very simplified and use round numbers for quick calculation.
The largest component of the body is water. Water makes up between 45 and 75% of body weight, with the variability due primarily to differences in body fat. While most tissues including muscle, skin, and visceral organs are over 70% water, adipose tissue contains less than 10% water. The percentage of body weight that is water therefore varies inversely with body fat. In the average lean adult male around 60% of the body weight is water. The remaining body weight consists of 16-18% fat with 22-24% protein, carbohydrate and other solids. In the female the percentage of body weight that is water is lower due to a relatively greater amount of subcutaneous fat.[1]
Body water is broken down into the following compartments:[2]
- Intracellular fluid (2/3 of body water)
- Extracellular fluid (1/3 of body water)
- Plasma (1/5 of extracellular fluid)
- Interstitial fluid (4/5 of extracellular fluid)
- Transcellular fluid (a.k.a. "third space," normally ignored in calculations)
- Contained inside organs, such as the gastrointestinal, cerebrospinal, peritoneal, and ocular fluids.
The simplest calculation is the 60-40-20 rule.
- Total Body Water = 60% of body weight
- Intracellular fluid = 40% of body weight
- Extracellular fluid = 20% of body weight
This is consistent with the above relations between total body water and the compartmental fluids.
These proportions are not preserved in fluid interventions, however. For example, pure water added to total body water will distribute throughout the above compartments, but isotonic saline will remain in the extracellular compartment.
[edit] Measurement of body water
[edit] Dilution and equilibration
Total body water can be determined using Flowing afterglow mass spectrometry FA-MS measurement of deuterium abundance in breath samples from individuals. A known dose of deuterated water (Heavy water, D2O) is ingested and allowed to equilibrate within the body water. The FA-MS instrument then measures the deuterium-to-hydrogen (D:H) ratio in the exhaled breath water vapour. The total body water is then accurately measured from the increase in breath deuterium content in relation to the volume of D2O ingested.
Different substances can be used to measure different fluid compartments:[3]
- total body water: tritiated water or deuterium
- extracellular fluid: inulin
- blood plasma: Evans blue
Intracellular fluid may then be estimated by subtracting extracellular fluid from total body water.
[edit] Bioelectrical impedance analysis
Another method of determining total body water percentage (TBW%) is via Bioelectrical Impedance Analysis (BIA). In the traditional BIA method, a person lies on a cot and spot electrodes are placed on the hands and bare feet. Electrolyte gel is applied first, and then a current of 50 kHz is introduced. BIA has emerged as a promising technique because of its simplicity, low cost, high reproducibility and noninvasiveness. BIA prediction equations can be either generalized or population-specific, allowing this method to be potentially very accurate. Selecting the appropriate equation is important to determining the quality of the results.
For clinical purposes, scientists are developing a multi-frequency BIA method that may further improve the method's ability to predict a person's hydration level. New segmental BIA equipment that uses more electrodes may lead to more precise measurements of specific parts of the body.
[edit] Fluid loss
Volume contraction is a decrease in body fluid volume, with or without a concomitant loss of osmolytes. The loss of the body water component of body fluid is specifically termed dehydration.[4]
Na+ loss approximately correlates with fluid loss from extracellular fluid (ECF), since Na+ has a much higher concentration in ECF than intracellular fluid (ICF). In contrast, K+ has a much higher concentration in ICF than ECF, and therefore its loss rather correlates with fluid loss from ICF, since K+ loss from ECF causes the K+ in ICF to diffuse out of the cells, dragging water with it by osmosis.
[edit] References
- ^ [1]
- ^ John T. Hansen, Bruce M. Koeppen, (2002). Netter's Atlas of Human Physiology. Teterboro, N.J: Icon Learning Systems. ISBN 1-929007-01-9.
- ^ Physiology at MCG 7/7ch02/7ch02p13
- ^ MedicineNet > Definition of Dehydration Retrieved on July 2, 2009
[edit] External links
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